I would like to thank Ana Calvo and Maria Aguiar for the mentorship of this study as well as for their helpful suggestions; José Frade and Jorgelina Carballo for their contribution with the analytical techniques. A special thanks for Cláudia Pereira from Instituto dos Museus e da Conservação; Regina Andrade from Santa Casa da Misericória do Porto; Elisa Soares from Museu Nacional Soares dos Reis and Carla Felizardo from Centro de Conservação e Restauro – UCP, for their contribution to the survey undertaken.
1The impregnation coatings applied to the reverse of canvas supports consisted of saturating the textile fibres with adhesive material of an impregnating nature – originally animal glues, and later, drying oils, resins, wax-resin mixtures, etc.
2The application of this treatment to the reverse of canvas paintings had several objectives, such as: direct protection of the textile support, as well as a preventive action; isolation or waterproofing (protecting the support from humidity, dirt and micro-organisms); consolidating the paint and ground layers; and structural reinforcement (consolidation of the textile fibres).
3Since the 17th century, impregnation and lining adhesive recipes were developed (BOMFORD; STANIFORTH, 1981). From these, the use of drying oils either separately or mixed with pigments, and the well-known and most used, flour paste and wax-resin, stood out. These adhesives systems became widespread in the 18th century and were applied directly to the canvas support, impregnating the textile fibres irreversibly (ACKROYD, 2002). Commonly these types of materials were found applied to the reverse of many canvases, not only as a lining adhesive, but also as a direct protection treatment of the support, consolidation, and as reinforcement for structural treatments.
4According to Westby Percival-Prescott by 1637, Theodore De Mayerne (1573-1655)had proposed, in his manuscript, a fish glue impregnation method in order to ‘revive’ paintings and to stabilize cases of flaking of the paint layers, derived from the weak adhesive strength of some grounds (VILLERS,1974). De Mayerne reported further application of a layer of fish glue with wheat flour or starch, moderately in liquid and strong. If necessary, a second layer should be applied, composed of heated linseed oil, in a slow burn, with umber pigment, until a consistency of syrup is reached (DE MAYERNE, 1620-1646). The mixture had to be brushed on and, after drying, it would allow the painting to be exposed on a humid wall without deteriorating (VILLERS, 1974).
5In 1798, the famous French restorer of the Napoleonic era, François-Toussaint Hacquin (1756-1832), invented a lining technique based on a solution with oily binding-media, composed by elemi and mastic resins, turpentine essence, white poppy oil, and lead white pigment (CHEVALIER, 2010).
6Jean-Francois-Leonor Mérimée (1757-1836), in his book dating from 1830, “De la peinture à l’huile (...)”, recommended a recipe for an oil-based lining adhesive – more common for paintings exposed to humid environments – which consisted of linseed oil binding medium, thickened through prolonged boiling process, then mixed with lead white and to which a small portion of red lead pigment could be added (MÉRIMÉE, 1830; MACARRÓN, 2002).
7In 1882, Willian Muckley (1829-1905), in his handbook for painters and students, advised the application of a coat of white lead, presumably bounded in oil, on the back of the canvases for protection against humidity and harmful gases: “In rooms where gas is used for lighting, any textile fabric which may be in them greatly suffers (…). The backs of oil-pictures on canvas are subject to the same influence. It must therefore be seen that unless they are protected, destruction must ensue” (MUCKLEY, 1882; CARLYLE, 2001). The preoccupation of the author is evident, regarding the conservation problems most frequently observed in the paintings at that time. As the reverses of the paintings were not protected, the supports absorbed humidity and pollutant gases from the environment, promoting the degradation of textile fibres, and undesirable reactions with the hydrophilic and lipophilic materials constituting the overlying layers.
8In order to obtain information concerning this practice in Portugal, Portuguese and Spanish painting treatises and historical restoration manuals were studied, such as the treatise of painting by Filipe Nunes (?-1655), the treatise of painting by Francisco Pacheco (1564-1644), the restoration manual by Vicente Poleró y Toledo (1824-1911), the treatise by Mariano de la Roca e Delgado (1825-1912) and the restoration handbook by Manuel de Macedo (1839-1921). Unfortunately these documents failed to provide specific information regarding this topic.
9A study was undertaken on an 18th century canvas painting, “The Knight of the Order of Malta”, belonging to the Ordem Terceira de São Francisco do Porto (TEIXEIRA 2014) (Fig. 1). In addition to the scientific study made of the materials and techniques and of the conservation and restoration treatment carried on the painting, an extensive investigation was simultaneously conducted to fulfill the lack of knowledge on ancient impregnations of canvas supports.
Fig. 1 The 18th century painting on canvas “The Knight of the Order of Malta” from the Ordem Terceira de São Francisco do Porto
Front view of the painting before and after the restoration and conservation treatment.
Credits: Andréa Teixeira.
10The painting was executed on a canvas support measuring 156 cm (height) x 150 cm (width), composed by two pieces of linen textile, joined by a seam in the horizontal direction.
11In the past, the reverse of this painting was subjected to several partial and total treatments, with the purpose of stabilising the structural damage of the textile support. It consisted of the application of an oily impregnating material of red brownish paint that covered the totality of the support to the point of the perimeter stretcher bar members. The lack of material below the stretcher bars must, therefore, indicate that the layer was applied after the canvas was already fixed to the stretcher. The impregnation is composed of two layers. The first is red in colour and it was applied directly to the surface of the textile support as a general structural reinforcement and to promote the adhesion of several thin rectangular shaped canvas patches, used to locally stabilise gaps. It was clear that over these patches a second impregnating coating was applied consisting of a dark brown colour. This layer was equally applied over the entire surface, covering and reinforcing that previously treated structure. Probably at a separate time, new patches of diverse dimensions and materials were applied (canvas, paper, newspaper), overlaying the aforementioned structure, with the intention to stabilize gaps and tears that occurred at separate later dates (Fig. 2).
Fig. 2 The reverse view of the painting on canvas “The Knight of the Order of Malta”
The red brownish oil impregnation layer and the patches applied in the past to the reverse of the canvas support. The graph on the right side shows: the red brownish oil impregnation (light pink colour); the thin rectangular canvas patches (greenish blue colour); a paper patch (blue colour); a newspaper patch (orange colour) and the canvas patches applied later on (pink colour).
Credits: Andréa Teixeira.
12In order to contribute to the identification and characterisation of the materials used in such practice, micro-samples from the impregnation layers on the reverse of the case-study painting were taken.For pigment and filler identification and for stratigraphy characterization, cross-sections embedded in polyester resin Technovit 4004 were examined by Optical Microscopy (OM) under visible reflected and polarised light and ultraviolet (UV) radiation, with an OLYMPUS BX41 microscope (objective magnification range from 100x to 200x). Digital images were recorded with an integrated OLYMPUS C-4040 Zoom digital camera/ProgRes® CapturePro 2.7. Examination was complemented with further analysis by Scanning electron microscopy coupled with an energy dispersive X-ray spectroscopy (SEM-EDXS). This was performed with a Jeol JSM – 6390 LV Scanning Electron Microscope, variable-pressure and Oxford Instruments Spectrometer software INCA X-Ray.
13Identification of binding material and organic pigments and dyes was obtained by gas chromatography mass spectrometry, on an Agilent 6890N gas chromatograph (GC) coupled with an Agilent 5973N mass spectrometer (MS). Prior to analysis the samples were derivatised with Met-Prep II (methanolic solution of m-trifluoromethylphenyl Trimethylammonium Hydroxide).
14Fourier transform Infrared spectroscopy (transmission and FTIR-ATR) was carried out with a Nicolet 6700 spectrophometer attached to a Continuum IR microscope equipped with MCT/A detector - Smart Orbit Diamond 30000 – 200 cm-1. The spectra was collected in transmission mode, with spatial resolution of 50-100μm, an optical resolution of 4 cm-1 and 128 scans, by using a Thermo diamond anvil compression cell.
15Microscopic examination of the cross-sections confirmed the presence of a double layer (Fig. 3): the first one, a thicker red layer, was applied directly to the support (140µm) and the second one, a thinner dark brown layer (15 µm), is situated directly above.
Fig. 3 Optical Microscopy (OM) under visible and UV radiation
The stratigraphy of the impregnation coating applied to the reverse of the painting. Visible red, reddish-orange particles with a few white particles, in the first layer. The yellowish fluorescence might indicate the presence of organic materials.
Credits: Andréa Teixeira.
16In the red layer, SEM-EDX spectrum shows peaks of iron (Fe) element which can indicate the presence of an iron oxide pigment. The red particles visible under OM can suggest the presence of red earth (Fe2O3), as a major pigment in the matrix. An iron oxide pigment, which mostly contains a significant concentration of potassium (K), aluminium (Al), silicon (Si), magnesium (Mg) and calcium (Ca) – all inorganic elements peaks detected in the EDX spectrum (Fig. 4). In addition to these elements, a large lead (Pb) peak and a medium one were also shown in the EDX spectrum. We can consider the presence of white (2PbCO3.Pb(OH)2) and red lead (Pb3O4) pigments in this layer, in a low concentration, shown by the few white and reddish-orange particles observable in optical microscopy, under visible light, respectively.
17Concerning to the dark brown second layer, the elemental information given by the SEM-EDX analysis (Fe, Si, Al, k, elements peaks) and the morphology of the pigments particles visible in the OM images have suggested the same red iron oxide pigment, as the predominant colour. Phosphorus (P) and Sulfur (S) peaks were identified, in low concentration, indicating the presence of bone black pigment (C+Ca3(PO4)2 + CaCO3). Lead white pigment was shown in this layer, in low concentration as well. Manganese (Mn) peaks associated with iron (Fe) element identify the presence of umber pigment (Fe2O3.H2O MnO2). The calcium (Ca) element detected in both EDX spectrum can indicate the presence of calcium carbonate particles on the recipe formulation.
Fig. 4 EDX spectrums from the red layer and the dark brown layer
EDX red layer spectrum revealing the presence of red iron oxide, white lead, red lead and calcium carbonate pigments. EDX dark brown layer spectrum identifying the presence of red iron oxide, white lead, umber, bone black and calcium carbonate pigments.
18The ratio between palmitic acid and stearic acid (P/S) serve as an identifying parameter for linseed, walnut and poppyseed oils. This can be determined with GC-MS analysis. P/S values included in the range 1.4-1.9, are characteristic for linseed oil; 2.4-2.9, for walnut oil; and 2.9-3.7, for poppy seed oil (PINNA, et al 2009). The chromatographic profile showed the presence of both saturated acids (palmitic acid and stearic acid) and the decomposition products of the oil paint film (azelaic acid) (table 1 and fig. 5). Linseed oil was identified after comparing the P/S ratio of a sample removed from the red layer. In the dark brown layer there was evidence of animal glue and, in low concentration, linseed oil and a diterpenic resin of the pinacea family (probably colophony).
19Peaks associated with bonds containing O-H, C-H, C-C and C-O functional groups were observed in the FTIR spectrum of a sample removed from this second layer. The combination of these bonds suggests the presence of starch molecules (Fig. 6). It is probable that this material is present in low proportions, in addition to linseed oil and a diterpenic resin.
Table 1 - Ratio between azelaic (Az), palmitic (P) and stearic (S) acids
Dark brown layer
Fig. 5 Chromatograms of the red layer and the dark brown layer
GC-MS analysis suggested the presence of linseed oil and animal glue as the major organic component in the red and in the dark brown layers, respectively.
Fig. 6 FTIR spectrum from the dark brown second layer
Starch functional groups shown in the FTIR spectrum.
20The condition the “The Knight of the Order of Malta”, before current treatment, was quite poor.
21The damages presented by the painting support included numerous losses and tears of several formats and dimensions. These were caused by the inability of the canvas to counter mechanical impact due to the brittleness of the fibres. Patches of heavy rigid canvas (impregnated with oil) were applied over damages in the past. These extend well beyond the damaged area. (Fig. 7). The original canvas and patch material have reacted to fluctuations in humidity in the surrounding environment differently, resulting in contraction, the creation of bulges and pronounced folds. The surrounding surface topography is consequently deformed (Fig.8).
Fig. 7 Canvas patches applied in the past in the reverse of the canvas support
Large canvas patches, applied to provied support to damages in the textile support. The double layer impregnation is visible.
Credits: Andréa Teixeira.
Fig. 8 Reverse view of the painting, under raking illumination
Canvas support deformation and pronounced folds near to the rigid canvas patches.
Credits: Andréa Teixeira.
22The restoration treatments performed previously, with the application of irreversible and acidic materials prone to polymerization, are the source for the conservation problems that the painting currently presents. The impregnation of the structure with materials that became tremendously hard and the adhesion of thick patches in the support, all contributed to the structural deterioration of the work.
23Regarding surface problems, the previous abrasive chemical cleaning of the original varnish, the presence of numerous oil repaint areas (that largely exceeded the limits of the paint film losses) and the application of a thick varnish layer applied afterwards, also contributed to the degradation of the painting. Although all constitute evidences of historical practices in the context of the conservation and restoration of painting on canvas, their degrading effects lead to a critical reflection on such practices.
24In order to document the practice of canvas impregnation in Portugal and to evaluate how far this practice was widespread, several museums and institutions were visited or contacted to gather information about paintings treated this way. An inventory was produced based on information collected at the Instituto José de Figueiredo (IJF) that provided information about national and regional museums. National museums, such as Museu Nacional Soares dos Reis (Porto) and local institutions as: Santa Casa da Misericórdia do Portoand Centro de Conservação e Restauro of the Portuguese Catholic University (UCP), also contributed to the survey.
25At the the Instituto José de Figueiredo (IJF) more than thirty paintings were found with impregnation coatings on the reverse. The application of those impregnation coatings were carried out in the past, by unknown conservators or artists.
26The conservators of the IJF revealed some important information about the condition and the conservation procedures performed by them to stablish the conservations problems that emerged later on. The damage and negative consequences that emerged on those canvases were directly caused by the inherent chemical processes that these impregnation layers undergo.
IJF conservators noticed that besides the impregnations treatments performed, several paintings had been submitted to glue paste lining as well, like the 17th centurypainting of S. Jerónimo, exhibited at the Monastery of Jerónimos, in Lisbon. This painting had a brownish impregnation coating on the reverse and its condition was very poor, as it presented a huge canvas support deformation due acid hydrolysis of the textile fibers and many paint film losses.
27The portrait D. Alonso de Quadros, from the 17th century belonging to the Palacete dos Malheiros Reymão (Viana do castelo) had a dark red impregnation coating all over the reverse (Fig. 9). The condition of the support was weak. The textile fibres were degraded due to acid hydrolysis chemical reaction, promoted by the interaction of the impregnations materials applied in the past. Deformations in the support emerged due to the strength of the adhesive used to apply the rigid patches. These are situated below the impregnation layer. During the last conservation treatment, carried out by conservators at the Centro de Conservação e Restauro in UCP (2012), the old impregnation layer was removed using a solvent gel. After the sensitive and cautious treatment, it was noticed that the canvas support regained some flexibility, despite the remnants of some of the impregnation material in the fibers. Total elimination was not possible because the material deeply impregnated the fibres.
Fig. 9 Front and reverse view of the canvas portrait D. Alonso de Quadros, dated to 17th century
Visible dark red impregnation coated all over the reverse surface. Devormatins in the canvas support deformation and folds near to the canvas patches are clearly visible.
Credits: Carolina Barata (lecture in UCP).
28The two paintings Conversão de S. Paulo and S. Jerónimo from the 19th century, in the collection of the Museu Soares dos Reis, exhibited impregnation layers of a brownish red colour, clearly applied during a past conservation treatment. A glue paste lining were observed in the painting S. Jerónimo (with the presence of fungal attack) applied above the impregnation coating. It is believed that the impregnation may has been applied with the purpose of waterproofing or direct protection of the reverse, as a moisture barrier.
29The Estigmatização de S. Francisco de Assis from the Casa da Juventude de Campanhã (Oporto) is an interseting 19th century example. This painting had a glue paste lining applied over a previous brownish red oil impregnation layer (Fig.10). Mould was detected on the reverse of the lining canvas. During the last conservation treatment, which removed tthe lining canvas, mould was not observed on the original canvas support. Probably due the presence of the oil impregnation that did not allow the fungi development.
Fig. 10 Front and reverse view of the canvas painting “Estigmatização de S. Francisco de Assis”, from the 19th century
Fungal growth is evident in the lining canvas. It is probable that the oil impregnation coating, applied prior to the glue paste lining, prevented the growth of mould in the original canvas.
Credits: Carolina Barata, lecture in UCP.
30The orange-reddish impregnation layer applied to the painting, Ecce Homo, from the end of the 16th century, belonging to the Santa Casa da Misericórdia do Porto, covered the entire reverse surface, even the reverse of the canvas which is covered by the stretcher (differing from the previous examples, which were applied while the canvases were stretched) (Fig.11). As it was applied before the painting being stretched, this particular practice could have been done whilst the painting was being executed as a preventive method – with the purpose of direct protection against moisture, damp, etc. As the painting did not show any signs of conservation and restoration treatments, the possibility to be used as a preventive method, should be considered.
Fig. 11 Front and reverse view of the canvas painting Ecce Homo, from the end of the 16th century
The orange-reddish impregnation layer applied over the entire reverse surface of the painting (even the tacking margins).
Credits: Regina Andrade (Santa Casa da Misericórdia do Porto).
31The portrait of D. João VI, from the 18th century had a dark brown-reddish impregnation treatment applied up to the limit of the stretcher bar members (Fig.13). Similarly to the above painting, its condition was reasonable, although it had a light deformation all over the canvas support.
Fig. 12 Front and reverse view of the canvas painting D. João VI, from the 18th century
The dark brown-reddish impregnation was applied up to the limit of the stretcher bar members.
Credits: Nancy Fonseca (Centro de Conservação e Restauro of the UCP).
32Both impregnation layers commented above were applied homogenously and were stable, with no evidence of conservations problens.
33Direct impregnations on the textile support may trigger undesirable chemical reactions in the cellulosic structure of the fibres. Materials, like animal glue and starch, are vulnerable to biological attacks when exposed to high levels of relative humidity (RH). Consequently, a biological attack to these materials will affect the textile fibres containing cellulose, which is a material equally sensitive to attacks by micro-organisms.
34Linseed oil and resins are materials of high acidic characteristics that, when embedded within the canvas fibres, will chemically interact with the cellulosic nature of the support. The denominated acid hydrolysis is an irreversible chemical and physical degradation process that can occur on the fibres of canvases when they are exposed to a high RH and in the presence of an acidic environment.The resulting acidification of the textile support and the resulting increasing rigidity also given by the polymerisation process of the siccative oil, can eradicate almost the entire flexibility of the fabric and not responsive to thermo-hygrometric variations.
35The problems arisen put at risk the chemical stability of the support and cause physical damages that are a challenge to current conservators. It is important to mention that, in more sensitive cases, and in order to gain back the original flexibility of the fabric, it is crucial to treat the deformations and eliminate the rigidness of the support, rendering the removal of the layers imperative and inevitable. But, that again pose challenges to professionals as impregnation layers are intrinsically embedded in the fibres, making removal quite difficult to achieve and the toxicity of some pigments, as lead based, also pose health risks.
36The methods and materials used in Portugal to impregnate canvas support to provide additional support or to protect textile supports from moisture were not well known prior to this survey. There was little information know about the materials that were used, the periods of time when such practice was common nor how often this occurred. These questions led to the current survey in collections in Portugal. The results of the survey show that impregnations practice applied on the reverse of canvases date from the 17th century and were applied as a precursor to lining processes. These were used when the degradation in the textile became evident.
37During the study, it was possible to relate several damages to this type of intervention, as the acid hydrolysis process, the increasing rigidness, the added weight to the textile support, and the occurrence of accentuated structural deformations.
The literature consulted allowed a more comprehensive knowledge of the assorted materials used in the past and helped to validate the results obtained by the scientific analysis performed on the impregnation double layer of the “The Knight of the Order of Malta” painting. These results revealed a set of diversified material that showed similarities with some of the adhesive recipes of linings, due the presence of drying oil and colophony resin. Possibly added to create a thicker layer that would be more resistant and would fill all the interstices of the textured canvas, pigments were added, such as iron oxides and the siccative pigments of umber, white lead and red lead. Those would accelerate the drying time of the oil binding and would provide protection against moisture.
38The study of paintings gathered in the survey led to some interesting findings, like the three cases where paste glue linings were found, associated to impregnation layers, applied before lining treatment. It is most likely that the impregnation was employed as an isolating layer and to protect the support against humidity, as the lining adhesive was aqueous.
39Other particular aspect was the noticeable preference given to red and brown pigments which granted a brownish red tone to the impregnations. Further identification of the pigments and fillers used through scientific analysis would contribute to characterise the nature of such impregnation layers, to establish relations between practice and recipes found in literature and to highlight the criteria for material selection (function, accessibility and cost).
40This subject topic remains open for further study, where an extended number of paintings could be analysed, in order to achieve further detailed results and, therefore have a more realistic approach of the restoration practice in Portugal.